1. Field of the Invention
The invention relates to isocyanate trimers containing iminooxadiazine dione groups, a process for their preparation and their use for preparing coatings, adhesives and plastics.
2. Description of the Prior Art
It is known to convert isocyanates by trimerization into isocyanurates (1,3,5-substituted hexahydro-s-triazine-2,4,6-triones; no data is given hereinbelow as to the degree of hydrogenation of the heterocycles such as "hexahydro", reference being made in a general manner to species having single bonds in the ring). The 1,3,5-triphenyl derivative, which is obtainable, for example, by trimerizing phenylisocyanate in the presence of potassium acetate, was synthesized for the first time in 1885 (A. W. Hofmann, Chem. Ber. 1885, 18, 765 et seq.). Although other methods are also possible for synthesizing isocyanurates (cf. H. F. Piepenbrink, "Houben/Weyl, Methoden der Organischen Chemie" 4th edition, Vol. VIII, Oxygen Compounds III, G. Thieme Verlag, Stuttgart, 1952, ed. E. Muller, p. 244 et seq.), the simplest way is still to trimerize isocyanates.
In particular isocyanurate polyisocyanates which are accessible as a result of trimerizing commercially available diisocyanates, such as tolylene diisocyanate (TDI), bis(isocyanatophenyl) methane and polyphenylene polymethylene polyisocyanates as prepared by aniline-formaldehyde condensation followed by phosgenation (MDI), hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) and bis(isocyanatocyclohexyl) methane (H.sub.12 MDI), have proven to be qualitatively high-grade raw materials, inter alia, for the preparation of polyurethane plastics materials and polyurethane coatings. Furthermore, trimerization is a conventional cross-linking reaction, in particular in the case of aromatic polyisocyanates, for preparing high molecular weight, optionally foamed, plastics.
These prior art systems exhibit some disadvantages. If, for example, diisocyanates are trimerized in order to prepare isocyanurate polyisocyanates which are viable, in particular in the lacquers and coatings sector, the (melt) viscosity of the resulting polyisocyanates is sometimes extremely high. This is particularly the case when working with high degrees of conversion or high resin yields, which may sometimes result in problems in working or utilizing these products.
On the other hand, however, a high degree of conversion is desirable for a number of reasons. For instance, there are important economic factors to consider because of the time- and energy-consuming separation of the monomer after the trimerization reaction, which is required from an environmental standpoint. Then there is the increase in the NCO functionality (f) of the trimers which is associated with the increasing degree of conversion of starting diisocyanate as a result of the formation of product constituents containing more than just one isocyanurate ring. This again is highly desirable because products having a high cross-link density and also high physical and chemical stability are obtained in this way. For the sake of simplicity these species will be characterized hereinbelow by the number of diisocyanate molecules, n, which are incorporated (n=3,5,7, . . . ). If n=3, f=3, when n=5, f=4, etc. However, as n increases so does the (melt) viscosity of the polyisocyanate trimers.
Consequently, in order to prepare low viscosity trimers either the reaction must be terminated at a very low conversion, in order to obtain as high a proportion as possible of "n=3" trimer in the mixture, or the "n=3" species is separated subsequently out of oligomer mixtures, optionally also only in enriched form (cf. DE-A 3,810,908; WO-A 93/07 183). Neither one of the two methods is advantageous in economic terms. Low conversion rates result in heavy losses in resin yield, which as previously indicated means a high economic cost, and regardless of the type of separation, the process necessarily results in a by-product of higher viscosity fractions in addition to increased process costs. Furthermore, it can be difficult when carrying out industrial trimerization to obtain reproducible uniform products if further reaction has to be interrupted even after a very short time (homogenization problems, incomplete side reactions and secondary reactions between co-catalysts which are frequently co-used, etc.).
A number of substances and processes have therefore been proposed to reduce the viscosity of lacquer polyisocyanates. One involves the use of reactive thinners, i.e., substances which exhibit a low intrinsic viscosity, normally below 300 mPa.cndot.s at 23.degree. C., and have groups which are capable of reacting with reaction partners of the polyisocyanates, for example, polyhydroxyl compounds. Polyisocyanates based on aliphatic diisocyanates (especially HDI) and containing uretdione groups ("dimers") and/or allophanate structure have been used for this purpose. (H. J. Laas et al., J. Prakt. Chem. 1994, 336. 196-198).
It is generally immaterial in terms of the final viscosity of the polyisocyanate mixture whether the mixture was produced by the simultaneous formation of the high viscosity isocyanurates and low viscosity allophanates or whether separately produced products are mixed subsequently.
Both uretdione group-containing and also allophanate polyisocyanates (provided that the allophanates have been obtained from diisocyanates and monoalcohols) are primarily difunctional. Allophanates based on higher functional alcohols exhibit no viscosity advantages over biuret polyisocyanates or isocyanurate polyisocyanates (DE-A 2,729,990). Regardless of which type of low viscosity reactive diluent is used, the functionality of the polyisocyanate mixture is lowered. To significantly lower the viscosity in HDI polyisocyanates, such high concentrations of difunctional reactive thinners are necessary that the functionality of the resulting mixture is already markedly below 3 (DE-A 19,603,736).
A further factor is that the uretdione four-membered ring is thermally unstable and dissociation into the starting diisocyanates takes place at elevated temperature. In the case of prior art low viscosity uretdione reactive thinners, which are obtained, for example, in accordance with DE-A 1,670,720 by the phosphine-catalyzed dimerization of HDI, this gradual cleavage to reform HDI monomer can begin in the drying cabinet at temperatures of above 60.degree. C.
To a lesser extent, in particular at temperatures above 150.degree. C., this thermal stability problem also applies to allophanates which dissociate into more thermally stable urethane and isocyanate groups.
Low viscosity aliphatic polyisocyanates having optimal functionality can also be produced by alternative reactions, for example, by reacting silylized alcohols with isocyanato-alkanoic acid chlorides (Ch. Zwiener, L. Schmalstieg, M. Sonntag, K. Nachtkamp and J. Pedain, Farbe und Lack 1991, 1052-1057 and bibliography).
The disadvantage here is that isocyanatoalkanoic acid chlorides are not available industrially and they can involve handling problems. The process is very costly which outweighs the anticipated product advantages, primarily the low viscosity of the polyisocyanates.
Another disadvantage of isocyanurate polyisocyanates is their compatibility with certain polyols that are not sufficiently polar (DE-A 3,810,908). This can result in restriction on use, for example in the lacquers and coatings sector. According to the teachings of DE-A 3,810,908, this disadvantage may be overcome by terminating the trimerization while conversion is still low, thus obtaining isocyanurate polyisocyanates having at least 60 wt. % 1,3,5-tris(6-isocyanatohexyl) isocyanurate. However, this method is not advantageous for economic reasons as previously discussed.
An object of the present invention is to provide polyisocyanates, which are qualitatively at least equivalent to the isocyanurate group-containing polyisocyanate products, and either do not suffer or suffer to a lesser extent from the disadvantages of the prior art products.
This object may be achieved with isocyanate trimers according to the present invention.